Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is referred to as an ion implantation recipe.
FIG. 1 depicts a prior art ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane 12). A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions.
In the design and operation of an ion implanter, ion dose uniformity and ion beam utilization are two major concerns since they directly impact the productivity of the ion implanter.
To achieve a uniform distribution of dopants, an ion beam is typically moved across the surface of the target wafer during an implantation process. FIG. 2A shows a typical setup for continuous implantation with an ion beam. In an ion implanter system, an ion beam spot 202 may be swept horizontally (i.e., in the X direction) across the surface of a wafer 204. A dose control Faraday cup 206 may be used to measure the ion beam current. At the same time, the wafer 204 may be translated vertically along a path 212 (i.e., in the Y direction) through a process chamber. Thus, the ion beam spot 202 is scanned with respect to the wafer 204 in both the X and Y directions. The net effect of the movement of the ion beam spot 202 in the X and Y directions is a beam path 20, as shown in FIG. 2B, that zigzags across the entire surface of the wafer 204 as well as its surrounding area. Since the ion beam spot 202 moves completely off the wafer 204 in each sweep, the total area covered by the ion beam spot 202 may be approximated with a box 22, which is much larger than the wafer 204.
The traditional implantation method as illustrated in FIGS. 2A and 2B has a number of problems, especially when a low-energy (thus low-current) ion beam is used. First, such a method often assumes, as illustrated in FIG. 3, that (1) the ion beam spot 302 is relatively small compared to the target wafer 304, and (2) the ion beam spot 302 maintains the same profile 32 and delivers the same dose 34 at any location. While these assumptions may be acceptable for medium- and high-energy ion beams, they are not applicable to low-energy ion beams. As illustrated in FIG. 4, a low-energy ion beam 402 often has a spot size comparable to the target wafer 404, and the beam profile 42 can vary drastically resulting in a non-uniform dose 44.
Second, in the traditional method, the ion beam spot goes completely off the wafer edge in each sweep, which is known as a “full overscan.” Full overscans are deemed necessary to provide a uniform ion dose even at the edges of the wafer and to allow real-time monitoring of the ion beam conditions. For example, in one known system, the ion beam spot is programmed to overscan by 1.1*W on one side and 1.5*W on the other, wherein W denotes a half-width of the ion beam spot (e.g., a distance from beam center to where beam current falls below 1/100 of its maximum). If the spot size is small (i.e., W is small), the ion beam is off the wafer surface only briefly. However, if the spot size is large (e.g., greater than about a quarter of the wafer size), as is often the case for low-energy ion beams, the ion beam spot spends almost as much, if not more, time off the wafer as it is on the wafer. As a result, beam utilization becomes extremely low for a low-energy ion beam that is scanned fully off the wafer.
In view of the foregoing, it would be desirable to provide a solution for ion implantation which overcomes the above-described inadequacies and shortcomings.